1. Field of the Invention:
The present invention relates to a suction-holding apparatus and a wafer polishing apparatus.
2. Description of the Related Art:
Wafer polishing apparatuses for polishing semiconductor wafers employ e.g., a polishing device having a polishing pad. By the polishing process, it is expected that the semiconductor wafer is to have a predetermined thickness, and also have a smooth surface. To this end, it is essential to hold the semiconductor wafer securely in a flat state. An example of suction-holding apparatuses for holding a semiconductor wafer is disclosed in JP-A-No. 2000-158268.
The suction-holding apparatus of JP-A-No. 2000-158268 includes a base and a suction plate. The base includes a circular cavity, and a plurality of suction holes formed in the bottom face of the cavity. The suction holes are connected to a suction device such as a pump, so as to suck an object.
The suction plate is fitted in the cavity of the base. The suction plate is composed of a plurality of annular elements that are concentrically disposed. The annular elements include high-permeability annular elements constituted of an air-permeable material such as a porous ceramic, and low-permeability annular elements that are barely air-permeable. The high-permeability annular elements and the low-permeability annular elements are alternately located. The low-permeability annular elements are significantly smaller in radial size than the high-permeability annular elements, and serve as blocking walls that restrict permeation of air.
When the wafer polishing apparatus is used for polishing, the semiconductor wafer to be polished is mounted on the suction plate by using the suction-holding apparatus. Then the semiconductor wafer is sucked through the suction holes. At this point, a suction force is exerted on the semiconductor wafer through the high-permeability annular elements of the suction plate. The suction force allows the semiconductor wafer to be held by the suction-holding apparatus, thus to be ready to be polished. In the suction-holding apparatus, the suction holes are divided into some groups, each of which is connected to the suction device. Controlling the suction performance of each group, for example by using valves, allows only selected high-permeability annular elements to activate the suction force according to the size of the semiconductor wafer to be polished, from those having a relatively small diameter to those having a relatively large diameter.
The suction plate, including a portion made of e.g. a porous ceramic, is fixed to the base, using an adhesive, for example. Specifically, the suction plate is bonded to the bottom face of the cavity of the base. However, the suction plate is repeatedly subjected to compressive force and shear force from the polishing device, via the semiconductor wafer. Accordingly, the bonding force of the adhesive declines through repeated polishing sessions, and one or more of the plurality of annular elements of the suction plate may be separated from the base. In such a case, a minute step is formed on the surface of the suction plate, thereby preventing the semiconductor wafer from being securely held.